In the automobile bearing used in the vehicle wheels, one of outer and inner members each having raceway surfaces for rolling elements, which serves as a rotatable member, is provided with a wheel mounting flange for the support of a vehicle wheel, and a rim of the vehicle wheel is fitted to this flange together with a brake rotor held in overlapping relation with the rim of the vehicle wheel. (See, for example, the Japanese Laid-open Patent Publication No. 2000-301401.)
The brake rotor, which is one of members that contact the bearing, is made of iron, which is a mainstream material therefor, but a strong demand has been made to reduce the weight of various component parts of an automotive vehicle in order to increase the mileage. Accordingly, attempts have been made to use the brake rotor made of an aluminum alloy. Considering that the aluminum alloy has a specific gravity that is about one third of that of steel, the brake rotor of the aluminum alloy is lighter in weight than the conventional brake rotor made of iron even if it is designed to have a thick wall structure in order to compensate for the shortage of rigidity as compared with steel. In the future, for reduction of the weight, the use of the brake rotor made of the aluminum alloy will increase.
However, the brake rotor made of the aluminum alloy and the steel rotatable member formed with the wheel mounting flange in the wheel support bearing tend to have a material relation to develop a metal-to-metal potential difference. For this reason, in the event that a surface of the rotatable member held in contact with the brake rotor is exposed to, for example, muddy salt water, an electric battery circuit tends to be formed between the rotatable member and the brake rotor, which eventually leads to electrode corrosion. Once the electrode corrosion occurs, sticking occurs in that contact surface, resulting in the workability during repair and/or inspection in the market being lowered. Although this electrode corrosion can be avoided when an insulating member is arranged between the rotatable member and the brake rotor, the number of component parts used increases and, for that reason, not only will the control of the component parts become complicated, but also mounting of the vehicle wheel onto the wheel support bearing will also become time-consuming and laborious.
On the other hand, a knuckle, which is one of the members that contact the bearing used for supporting the vehicle wheels, is widely made of an aluminum alloy for the purpose of weight reduction. A problem associated with corrosion resulting from formation of the battery circuit is similarly found between the knuckle made of the aluminum alloy and the outer member of the wheel support bearing. To cope with this problem, it is suggested to apply, as a coating treatment excellent in electric corrosion prevention, a Zn—Ni plating and a chromate treatment using hexavalent chromium to a surface of the wheel support bearing that contacts the knuckle. See, for example, the Japanese Patent Application No. 2002-266051.
However, the chromate used in the practice of the conventional chromate treatment for the prevention of the electric corrosion contains hexavalent chromium, which is an environmentally harmful substance. With the environmental concern rising in recent years, efforts have been made to restrict the use of hexavalent chromium. By way of example, the hexavalent chromium will no longer be used according to the European environmental regulation.